Van atta array



' Feb. 17, 1970 B. Ew.s 3,496,570

' VAN AT'I'A ARRAY Filed March 28, 196'? INVENTOR BERNARD L. LEWISATTORNEYS United States Patent US. Cl. 343754 Claims ABSTRACT OF THEDISCLOSURE A dielectric slab is placed or disposed in adjoiningrelationship to the line or plane of antenna elements of an otherwiseconventional Van Atta array, the dielectric constant and thickness ofthe slab selected to produce reflection and refraction of portions ofthe wave incident on the array to cancel waves specularly reflected bythe antenna elements of the array, without deleteriously affecting theretrodirective reradiation to be performed by the array. Alternatively,the surface of the array 1s curved or stepped, without use of adielectric slab, thus increasing the path lengths of certain of the Waveportions in air, which is compensated for by concomitant reductioninlengths of the respective transmission lines interconnecting theantenna elements of the array. Accordingly, there is no substantialeifect on the retrod1rective reradiation of incident signal by thearray, but What would otherwise be specular reflection is diifused bythe array surface.

I 7 Background of the invention The present invention relates generallyto methods and means for eliminating or substantially reducinginterference from specular reflection in Van Atta arrays. V I

Generally speaking, the Van Atta array or reflector 1s a form ofelectromagnetic wave reflector comprismg a plurality of antenna elementsdisposed in a symmetrical array relative to the geometric center,symmetrically arranged pairs of the antenna elements beingcoupled orinterconnected by appropriate transmission lines in terms of the type ofantennas employed, of equal electr cal length, to produce retrodirectivereradiation (or backreflection) of electromagnetic waves incident on thearray, with relatively broad-angle coverage in comparison to aconductive reflecting sheet of the same area. In essence, wave portionsor wavelets along a phase front of an incident electromagnetic wavewithin the angle of coverage of the array are received or absorbed bytheantenna elements in their respective paths, converted to or guided aselectrical energy which is fed through the associated transmissionlines, and reradiated as a reflected wave from the connected antennaelements of the pairs back in the direction from whence it originated.The equality of the path lengths of the wave portions between incidentand reflected fronts of the wave is such that the reflected energy isreinforced along the phase front.

The Van Atta principle iswell known and thoroughly discussed in theliterature; hence, further discussion of basic concepts is unnecessaryhere. For a broader treatment of the underlying theory of operation andexemplary construction, reference is made to United States LettersPatent 2,908,002, entitled Electromagnetic Reflector," granted Oct. 6,1959 to Lester Van Atta.

Like other receiving antenna arrays, however, Van Atta reflectorsproduce some scattering of the incident wave or signal back into space.Referring for example to a paper entitled A Van Atta ReflectorConsisting of Half Wave Dipoles by J. Appel-Hansen in IEEE Transactionsof Antennas and Propagation, vol. AP-l4, No. 6, November 1966, pp.694-700, it is observed by the author that presently known Van Attareflectors suffer from interference between the signal scattered fromthe array and that which is reradiated from the array via the cablecouplings between the antennas. Considering the effect of a plane waveincident from an arbitrary direction on a linear Van Atta reflectorconsisting of four parallel half- Wave dipoles (which, incidentally, arethe most often selected antenna elements for a Van Atta array) arrangedin two pairs, the author notes that each dipole is influenced by theplane wave and, in addition, by an excitation attributable to energyabsorbed from the wave by the connected dipole and transmitted via thetransmission line therebetween. From this he points out that the currentdistribution in each dipole is composed of two parts, one of which isthat current induced by the incident wave, and the other part thecurrent owing to the presence of the connected dipole. The lattercurrent is that producing the fields responsible for the wave reradiatedin the retrodirective direction, i.e., the Van Atta eifect. Thefirstmentioned current component, however, produces 'a scattered field,which in the case of half-wave dipoles is of substantially the samemagnitude as the retrodirective wave field, causing specular reflectioninterfering with the 'retrodirectively reradiated wave, most stronglywhen the plane wave is incident from the direction normal to the line orplane of the array. More particularly, the scattered signal can causesubstantial cancellation of the retrodirective signal. Another generallydeleterious eifect is the result of coupling between the antennaelements. The author concludes that considerable desirablebackreradiation is produced where proper selectionof (but still equal)lengths of transmission lines connecting the antenna elements is made.

As previously stated, all receiving arrays scatter some of the incidentradation back into space, and the Van Atta array produces scatteringirrespective of the particular type of antenna element employed,although where halfwave dipoles are used the effect is more pronounced.Accordingly, it is a principal object of the present invention toprovide means for preventing specular reflection from any Van Attaarray.

It is another important object of the invention to prevent propagationof specularly reflected signal or of a component thereof, in theretrodirective direction from a Van Atta reflector.

Still another significant object of my invention is to increase theangle of coverage of Van Atta arrays.

Summary of the invention Briefly, according to the basic concepts andprinciples of my invention means are provided in the otherwiseconventional Van Atta array for eliminating or reducing the undesirablespecular reflection from the array.

In one general form of the invention, the aforementioned means comprisesa dielectric medium having two parallel surfaces one of which abuts oris placed in adjoining or adjacent relationship to the surface or lineof the array either coincident therewith or generally parallel thereto,preferably along the plane containing the specular phase center of thearray. The dielectric constant of the medium is selected to producereflections of incident signal of equal magnitude and angle ofreflection from the interface or boundary between the dielectriccontiguous with the array and the medium in which the array is immersed,generally air. One of the reflected rays is a direct result of theair-dielectric interface while the other ray is one which has penetratedthe dielectric and has thus undergone refraction and specular reflectionfrom the surface of the array. The thickness of the dielectric betweenthe opposite surfaces thereof is selected to render these two reflectedrays of opposite phase, so that one cancels or substantially cancels theother at the air-dielectric interface.

In another form of my invention the aforementioned specular deflectionreducing means comprises an arrangement of the antenna elements of thearray in stepped or curved configuration so that the surface of thearray deviates progressively, on either side of its geometric center,from a flat planar or straight line surface, and a reduction in lengthof interconnecting transmission lines corresponding to the increase inpath length through the air. Accordingly, the array surface reflectivelydiffuses signals which would otherwise be specularly reflected at leastin part in the retrodirective direction, and hence substantially reducesinterference with and cancellation of desired reradiation in the backdirection, while acting as a flat array to the incoming wave producingenergy fed through the transmission lines interconnecting the antennaelements.

Brief description of the drawings The above and still further objects,features and attendant advantages of the present invention will becomemore apparent from a consideration of the following detailed descriptionof certain preferred embodiments thereof, especially when taken inconjunction with the accompanying drawings, in which:

FIGURE 1 is a symbolic schematic diagram of one general form of myinvention;

FIGURE 2 is a perspective view of an embodiment of the general form ofmy invention depicted in FIGURE 1; and

FIGURE 3 is a plan view of a Van Atta array incorporating a further formof my invention.

Description of the preferred embodiments Referring now to FIGURE 1, theprinciples contributing to one aspect of my invention are based, atleast in part, upon difference in index of refraction between materialsof different dielectric constant. In particular, the desired result ofpreventing the existence of a specularly reflected signal depends onreflection of rays (alternative referred to as waves or beams) from theinterface or boundary between different dielectric media and onrefraction of a ray within a dielectric medium.

As shown in FIGURE 1, the Van Atta array, which may be linear or planar,for example, or may be of other known or proposed simple or complexgeometrical configurations such as circular, cylindrical, spherical,tetrahedral, cubic, rectangular, and so forth, and which is symbolicallydepicted by the box appropriately labeled and designated by referencenumeral 10, is covered by a slab 12 of dielectric material. For the sakeof simplicity array is assumed to be linear and the dielectric slab 12is contiguous with the line of antenna elements of the array (or surfaceof the array), adjoining the elements along the plane 15 containing thespecular phase center.

The antenna elements of Van Atta array or reflector 10 are arranged insymmetrical pairs connected by equal lengths of appropriate transmissionline, in conventional or known fashion; the entire configurationdiffering from any typical Van Atta array only in the placement ofdielectric slab 12 against the antenna elements such that incomingelectromagnetic waves are incident first on the slab. The dielectricconstant k of the slab or plate is preselected in accordance with knownprinciples of electromagnetic and optical theory to produce a reflectedwave or wave portions from the air-dielectric interface (designated byreference numeral 18) whose strength or magnitude is at leastapproximately equal to the wave or wave portions reflected from thearray itself.

With reference to the figure, for a ray 20 incident at an angle irelative to the normal to the array and which is to undergo refractionin the dielectric medium 12 at an angle 1' to the normal, and thence isspecularly reflected from the array, again at the angle r to the normal,and is finally refracted at the air-dielectric interface at the angle1', the relationship between these angles and the dielectric constant kof slab 12 is sin r i The thickness 1 of slab 12 is also preselected,such that the phase of ray 22, which penetrates the air-dielectricinterface 18 and is reflected from the array 10, is opposite that of ray21 which is directly reflected from the airdielectric interface.Accordingly, since the ultimate direction of the two rays (21 and 22) isthe same, and they are of opposite phase, they tend to cancel each otherout. To this end, the thickness 1? of the dielectric plate is preferablyone-quarter wavelength of the incident wave in the dielectric of whichthe plate is composed. Mathematically,

where A is the free space wavelength of the wave. Alternatively, t ischosen to have as small a value as possible under the particularconditions that may be encountered in practice, to obtain the greatestbandwidth for the resultant improved array.

As an added advantage of the technique employed in the system of FIGURE1, the refraction of the incoming wave at the air-dielectric interfaceeffectively reduces the angle of incidence i on the elements of thearray to the angle of refraction 2', thereby increasing or broadeningthe agle of coverage of the array. It will be observed, for example,from Expression 1, above, that r is less than i for values of kexceeding unity (the dielectric constant of air), and that in suchinstances the dielectric-covered array is capable of reradiatingincident signal over a greater angle than that of which a conventionalVan Atta array is capable. Moreover, the cancellation of specularreflection transfers the power of the cancelled wave to the normalback-reradiated signal of the array, thus increasing array efficiency.

An exemplary embodiment of the improved array of FIGURE 1 is shown insomewhat greater detail in FIG- URE 2. In the configuration illustratedin FIGURE 2 the dipole antennas are placed in Van Atta array on thesurface of (or are embedded in the surface of) dielectric slab 33, suchas by conventional deposition techniques. A ground plane (conductivereflecting sheet) is disposed on the opposite face of the slab and pairsof symmetrically disposed dipoles (relative to the geometric center ofthe array) are connected by equal lengths of transmission line 37, 38,39.

By way of example only, without any intent to restrict the scope of theinvention, the dipoles are selected to be A (2 /k) in length and arespaced A /2 apart. The dielectric constant of slab 33 is selected ask=2.5 (suitable materials being Plexiglas, rutile, pyrite, amorphousselenium, germanium, or strontium titanate, to name a few),

and the thickness to be t== (4 /2.5).

The wave specularly reflected from the array is cancelled by waveportions of substantially equal magnitude and opposite phase which havepenetrated and emerged from the dielectric. That is, the wave portionssubsequently cancelling the specular reflections are refracted uponentry into dielectric plate 33, reflected from ground plane 35 andrefracted again at the dielectric-air interface.

Referring again to the more general case of FIGURE 1, when t is chosento be of small magnitude (e.g., A /(4 /k) and kzZ, the specifiedtechnique of matching to produce cancellation of specular reflection iseffective over a large range of angles of incidence because ofrefraction effects. If, for example, k has the value 2 and i is lessthan 45 degrees, then r is less than 30 degrees (from Expression 1 andthe maximum variation in path length through the dielectric with changesin i is AL 2V5 1)- 0.134 (3) In that event the maximum phase error(i.e., deviation from opposite phase) for ray 22 is 24 degrees when itcombines with ray 21, so that partial (substantially complete)cancellation is still achieved, since the resultant specularly reflectedray has less than one-quarter the power of either of the combined raysalone.

Referring now to FIGURE 3, an alternative embodiment of an improved VanAtta array capable of eliminating or substantially reducing specularreflection in the direction from which the incident wave emanated,includes antenna elements 50, such as dipoles, which are disposed inrelatively displaced configuration to produce a curved or stepped arraysurface. Accordingly, the array surface deviates from that of a linearor flat planar array as indicated by dotted line 52, so that extra pathlengths 55, 56, for example, in air are presented to the incoming waveportions.

The continuous curvature or discretely stepped character of the arraysurface is compensated for, insofar as the retrodirective reradiationresponse of the array is concerned, by reduction of the respectivetransmission line length by an amount equal to the extra path length inair. Thus, for example, transmission line 58 is reduced in length by anamount equal or approximately equal to twice the length of path 55. Itfollows, of course, that the transmission line lengths interconnectingpairs of symmetrically disposed antenna elements are no longer equal.The result is that the array is operative as a flat plate for normalretrodirective reflection of signal (since equal path lengths for signalwave portions are encountered just as in the standard Van Atta array),but specularly reflects in the manner of a diffusing surface, therebypreventing or reducing retrodirective specular reflection.

A ground plane 62 may be disposed behind the surface of the array, ifdesired, to ensure complete diffusion of specular reflection of incidentwaves.

I claim:

1. A passive electromagnetic reflector for retrodirectively reradiatingincident electromagnetic waves while eliminating or substantiallyreducing interference from scattered fields accompanying incidence ofsaid waves on said reflector, comprising a conventional Van Atta arrayof antenna elements which are symmetrically disposed in pairs about aselected geometric center of the array, each of said symmetricallydisposed pairs of elements interconnected by a separate transmissionline; and means including a dielectric member for varying at least oneconventional physical characteristic of the surface of said array andthe electromagnetic path lengths for incoming wave components whichwould otherwise result in said scattering fields.

2. The invention according to claim 1 wherein. said separatetransmission lines are of equal electromagnetic path length, and saiddielectric member is of dielectric constant exceeding that of thesurrounding medium, said dielectric member having a pair of parallelsurfaces one of which is contiguous with the surface of said array andhaving a predetermined dielectric constant to produce specular reflectedcomponents attributable to the direct reflection of incident wavecomponents from said interface and to reflection of incident wavecomponents from the array surface after penetration of said dielectricmember, which are of substantially equal magnitude and direction, thethickness of said member between said parallel surfaces beingpreselected to produce opposite phasing of said reflected components.

3. The invention according to claim 1 wherein said dielectric member iscontiguous with the surface of said array and has substantially uniformthickness in the direction normal to said surface of said array, saidthickness selected to be approximately equal to or less than a quarterwavelength of the electromagnetic waves to be reradiated, in thedielectric medium of said member.

4. The invention according to claim 3 wherein said dielectric member isdisposed behind the surface of said array relative to incomingelectromagnetic waves to be reradiated by said array.

5. The invention according to claim 4 wherein at least some of saidantenna elements are disposed on a surface of said dielectric member,and a conductive reflecting sheet is disposed against the oppositesurface of said dielectric member.

6. A passive electromagnetic reflector for retrodirectively reradiatingincident electromagnetic waves while eliminating or substantiallyreducing interference from scattered fields accompanying incidence ofsaid waves on said reflector, comprising a conventional Van Atta arrayof antenna elements which are symmetrically disposed in pairs about aselected geometric center of the array, each of said symmetricallydisposed pairs of elements interconnected by a separate transmissionline; and means varying at least one conventional physicalcharacteristic of the surface of said array and the electromagnetic pathlengths for incoming wave components which would otherwise result insaid scattering fields; wherein said means comprises an arrangement ofsaid antenna elements such that the surface of said array deviatesprogressively from a line perpendicular to the normal to the arraysurface at said geometric center to either side of said center, saidseparate transmission lines deviating from equal electromagnetic pathlength by respective amounts equal to the distance in said surroundingmedium from the antenna elements interconnected thereby to saidperpendicular line.

7. In a Van Atta array in which pairs of antenna elements symmetricallydisposed about a geometric center of the array are interconnected byrespective transmission lines, for retrodirective reflection of anelectromagnetic wave from incident on the array, the improvementcomprising means altering the electromagnetic path lengths of suflicientcomponents of the incident wave front to produce oppositely phasedsubstantially equal amplitude components relative to specularlyreflected components of the incident wave front to substantially cancelsaid specularly reflected components while maintaining equal pathlengths, and hence, substantial reflection without cancellation, forthose components of the incident wave front that would normally undergoretrodirective reflection.

8. The invention according to claim 7 wherein said means comprises adielectric member having a dielectric constant exceeding that of air,said member positioned adjacent the surface of said array and having athickness selected to produce said opposite phasing by refraction ofcomponentsof said wave front in conjunction with reflection ofcomponents of said Wave front from the surface of said array.

9. The invention according to claim 8 wherein said dielectric member hasa thickness of substantially one quarter wavelength of the incidentwave.

10. The invention according to claim 7 wherein said means comprises aprogressive rearward deviation of said antenna elements from a planararray to either side of said geometric center, said transmission linesdeviating from equal lengths by respective amounts equal to saidprogressive deviation of each of the antenna elements connected by therespective transmission line.

References Cited UNITED STATES PATENTS Iarns 343-777 Van Atta 343776Hannan 343776 X Malech 343-754 US. Cl. X.R.

